The present invention relates to improved green strength properties of elastomer blends. More specifically, the present invention relates to the incorporation of an ionogenic compound into a synthetic elastomer wherein the ionogenic group is pendant from the backbone of the synthetic elastomer so that, upon combining with an ionizable metal base or salt, the green strength of elastomer blends of the synthetic elastomer with either natural or synthetic cis-1,4-polyisoprene is improved.
Science and technology in the elastomer field has improved to such an extent that synthetic elastomers have supplemented or replaced natural rubber to a great extent in the fabrication of tires and other rubber products. Stereo-specific polymers and particularly synthetic cis-1,4-polyisoprene have demonstrated physical properties similar to natural rubber and, thus, are capable of becoming a complete replacement for natural rubber. However, a major deficiency of synthetic elastomers, including synthetic cis-1,4-polyisoprene, is its lack of sufficient green strength required for satisfactory processing or building properties as in building tires. The abatement of this deficiency has long been sought by the art and would greatly facilitate the replacement of natural rubber which is solely produced in tropical climates.
The term "green strength," while being commonly employed and generally understood by persons skilled in the rubber industry, is nevertheless a difficult property to precisely define. Basically, it is that property of an unvulcanized polymer common in natural rubber which, under normal building conditions where multiple components are employed, results in little or no unwanted distortion of any of the assembled components. Thus, with synthetic polymers or copolymers, adequate green strength, that is the requisite mechanical strength for processing and fabricating operations necessarily carried out prior to vulcanization, is lacking. That is, generally the maximum or "peak" stress which the unvulcanized materials will exhibit during deformation is rather low. Thus, unvulcanized strips or other forms of the elastomer are often distorted during processing or building operations. Although numerous additives and compounds have been utilized in association with various elastomers and particularly with synthetic cis-1,4-polyisoprene, substantial improvement in green strength has generally not been accomplished.
Green strength has generally been measured by stress/strain curves of unvulcanized compounds. Usually, the green strength of a compound is indicated by various properties of the stress/strain curve; typically, the average slope beyond the the first peak or inflection of the curve, the (ultimate) tensile strength, and the ultimate elongation. Improvements in any one or more of these stress properties indicate improved green strength.
Among the various additive compounds or agents which have been utilized to improve green strength of synthetic rubber elastomers are numerous nitroso compounds as set forth in U.S. Pat. Nos. 2,457,331; 2,477,015; 2,518,576; 2,526,405; 2,540,596; 2,690,789; and 3,093,614. Additionally, various dioxime compounds have been utilized such as those set forth in U.S. Pat. Nos. 2,969,341; 3,037,954; and 3,160,595 as well as in British Patent No. 896,309. Yet another class of additives or compounds are the diesters of 5-norbornene as set forth in U.S. Pat. Nos. 3,817,883 and 3,843,613.
U.S. Pat. No. 3,898,983 to Brancaccio relates to improving the green strength of polyisoprene by reacting maleic acid therewith. A similar patent is that of Yamauchi et al, U.S. Pat. No. 3,897,403, which relates to a reaction between synthetic cis-1,4-polyisoprene and maleic anhydride. These patents are clearly different from the present invention in that applicants' compound is not incorporated in synthetic or natural cis-1,4-polyisoprene, but rather in a synthetic elastomer which is then blended with natural rubber or synthetic cis-1,4-polyisoprene.
Another prior art patent which relates to improved green strength is French Pat. No. 2,215,429 which utilizes very small amounts of various carboxylic acids with various polymers such as polybutadiene and SBR. However, at higher amounts of the carboxylic acids, the rubbers are rendered unprocessable. This patent does not relate to blends containing any cis-1,4-polyisoprene (natural or synthetic) which is often needed for its cured properties such as low heat buildup. Furthermore, at carboxylic acid concentrations which lead to processable rubbers, the improvement of properties is small.
In an article by Brown and Gibbs, it is disclosed that unsaturated carboxylic acids were copolymerized with olefins and dienes wherein at least 100 milliequivalents of the acid were utilized, Rubber Chemistry and Technoloby, Volume 28, Page 938 (1955). However, essentially, thermoplastic rubbers were produced which readily reacted with zinc oxide or other polyvalent metal compounds to form crosslinks which could not be readily worked on mills or in internal mixers using classical mixing processes for the preparation of rubber compounds. Additionally, this reference relates only to the use of dienes or olefins and contains no suggestion whatsoever of natural or synthetic cis-1,4-polyisoprene or of elastomer blends. It is moreover stated that "the physical properties of an unvulcanized carboxylic elastomer having a carboxyl content of 0.1 equivalent or less were essentially those of an analogous, noncarboxylic polymer." However, the present invention finds this statement to be untrue.
In various articles published in the magazine, Soviet Rubber Technology, very small amounts of carboxylic acids were utilized in a manner similar to that set forth in the French patent noted above. Specifically, in articles by Kovalev et al, Volume 31 (5), Page 4 (1975); Marandzhera, Volume 30(2), Page 51 (1971); and Smirnov, Volume 30(6), Page 3 (1971), isoprene rubbers as well as butadiene-styrene thermoplastic block copolymer rubbers were produced containing carboxyl or ester groups. In general, these rubbers showed improved green strength. However, the introduction of carboxyl or ester groups was usually carried out at a pressure of about 250 atmospheres of carbon monoxide, a highly toxic gas. Additionally, the carboxyl or ester groups are introduced into an already preformed polymer such as polyisoprene or butadiene-styrene. Also, none of these references relates to blends of synthetic elastomers with natural or synthetic cis-1,4-polyisoprene or of elastomer blends.
In an article appearing in the Journal of Polymer Science, Volume 8(6), Pages 599-605 (1952), elastomers are formed utilizing carboxylic acids and esters thereof. These compounds, when cured, have exceedingly high modulus at 300 percent extension but are totally unsuitable to mill. Furthermore, no green strength improvement was reported and no data was reported for uncured properties. This reference also is solely related to copolymers of butadiene, and not to blends including natural or synthetic isoprene rubbers. The thrust of the entire reference was towards improved oil resistance and low temperature properties.
U.S. Pat. No. 2,880,186 to Barth relates to dipping products such as gloves into a latex solution so that a strong elastic film is formed. The technique described by Barth is used to improve the tear resistance of elastic films. Thus, a metal is incorporated in some soluble form, for example, a metallic resinate. Moreover, the enhanced tear resistance is achieved by condensing carboxylic acid groups contained in the latex polymer either during or after deposition of a film with a polyacidic cation or basic radical on the shaped article. Barth furthermore, as in Table I, actually shows a decreased ultimate tensile strength in the vulcanizate upon the addition of the butadiene-acrylonitrile-methacrylic acid terpolymer; this certainly would not lead one skilled in the art to expect the result of the present invention of actually increasing the uncured tensile strength.
U.S. Pat. No. 3,429,952 to Nordsiek et al relates to a compound wherein the only improved property is that of increased tear resistance, Column 3, lines 35 through 66 with all of the other properties, including elasticity, being subject only to very minor changes. Nordsiek recognizes no improved green strength properties. Moreover, the amount of unsaturated carboxylic acid is well in excess of applicant's recited range.
U.S. Pat. No. 3,475,362 to Romanick et al relates to a rubber-based adhesive containing a carboxyl additive, an organo-metallic compound, and a solubilizing resin along with the use of standard rosins. Since this patent relates to an adhesive system, the organo-metallic material furthermore is incorporated in some soluble form. Very importantly, the adhesive or mastic before cure is totally unsuitable for making into a sheet of uncured (i.e. green) rubber usable in tire building and the like.